XML schema support?

There is a need to support XML schemas in a similar manner to JSON ones. Some ideas for discussion:

• Most likely XML Schema, in the form of XSD files, would be the schema format.
  • By default, schema.xsd would be detected (like schema.yaml or schema.json).
  • Should we re-use the schema keyword in bblock.json or define a new one (e.g., xmlSchema)? Reusing would mean implementing some kind of autodetection (trial-and-error?) mechanism to tell XML and YAML/JSON files apart (e.g., when the URL does not have an extension).
  • schemaLocations in xs:imports could use bblocks:// URIs.
  • What would dependency detection look like?
    • <xs:import schemaLocation="...">
    • What else?
• XSLT could be used for XML to RDF transforms.
  • Would it be even possible to make it work the same way as JSON-LD contexts (i.e., with some kind of nesting)?
  • I think XML schema annotation (like we do in JSON schemas) wouldn't make sense.
  • XSLT > 1 support is not great outside of Java. There are a few projects that wrap the Saxon/C library from Saxonica, but only the FOSS version (which may be enough, though).

Related links:

• https://www.w3.org/community/rax/wiki/XML_to_RDF_Transformation_processes_using_XSLT

CC @rob-metalinkage @pzaborowski